1. Field of the Invention
The present invention relates to an image processing apparatus and an external storage unit used therein. More particularly, the present invention relates to an image processing apparatus intended for application to a television game apparatus and capable of displaying a color image on a screen of a display according to color data for each dot.
2. Description of the Prior Art
One example of an image processing apparatus of this kind is disclosed in Japanese Patent Laid-open (Kokai) Sho 59-118184 laid open on Jul. 7, 1984 (corresponding to U.S. Pat. No. 4,824,106 issued on Apr. 25, 1989). This prior art image processing apparatus displays a color image by combining a moving picture (objects) and a background picture (still images) on a screen of a raster scan monitor. Priorities are assigned respectively to the moving picture and the still picture, and only the moving picture or the still picture having a higher priority can be displayed in an area in which the moving picture and the still picture are superposed. In such a case, since the moving picture or the still picture having a lower priority is not displayed, it was impossible to display a state where two pictures are overlapped or a picture of a transparent object such as glass.
The foregoing prior art image processing apparatus may be able to display a moving picture and a background picture apparently overlapping each other by displaying the moving picture in every other frame of the raster scan monitor. However, even this displaying method has been unable to display a transparent object.
It is possible to display a transparent object if the image processing apparatus is provided with a memory for storing modified color data representing a superposed image of a moving picture and a still picture separately from original color data. In a computer graphic technology, it is possible to employ this method that stores additional color data beforehand because there is no restriction on a storage capacity. However, a television game apparatus must be inexpensive and hence its storage capacity is limited. Therefore, it is impossible to apply the computer graphic technology to the television game apparatus.